Testing mechanisms of Bergmann's rule: phenotypic but no genetic change in body size in three passerine bird populations

Bergmann's rule predicts a decrease in body size with increasing temperature and has much empirical support. Surprisingly, we know very little about whether 'Bergmann size clines' are due to a genetic response or is a consequence of phenotypic plasticity. Here we use data on body size (mass and tarsus length) from three long-term (1979-2008) study populations of great tits (Parus major), in which there has been a temperature increase, to examine mechanisms behind Bergmann's rule. We show that adult body mass decreased over the study period in all populations and that tarsus length increased in one population. Both body mass and tarsus length were heritable and under weak positive directional selection, predicting an increase, rather than decrease, in body mass. There was no support for micro-evolutionary change and thus the observed declines in body mass were a result of phenotypic plasticity. Interestingly, this plasticity was not in direct response to temperature changes but seemed to...伯格曼法则（Bergmann's rule）预测生物体型随温度升高而减小，且该理论已获得诸多实证支撑。令人意外的是，学界对“伯格曼体型梯度（Bergmann size clines）”的形成机制究竟源于遗传响应还是表型可塑性（phenotypic plasticity）却知之甚少。本研究借助1979-2008年间长期监测的三个大山雀（Parus major）种群的体型数据（体重与跗跖长）——这些种群的栖息环境均出现了温度上升——以探究伯格曼法则背后的作用机制。研究结果显示，所有种群的成体体重在监测期内均呈下降趋势，而其中一个种群的跗跖长有所增加。体重与跗跖长均具有可遗传性，且受到微弱的正向定向选择，该选择本应导致体重上升而非下降。本研究未发现微进化（micro-evolutionary）变化的相关证据，因此观测到的体重下降实则为表型可塑性的结果。有趣的是，这种可塑性并非直接响应温度变化，而是似乎……